1. Field of the Invention
The present invention relates to the field of telecommunications and in particular to optical transmission without any physical carrier. Still more in particular it relates to a method and system for establishing a terrestrial laser communication link capable of compensating for atmosphere scintillation, for instance caused by wind and/or turbulence.
2. Description of the Prior Art
It is known to use laser beams for transmitting information-carrying signals through the air and/or vacuum. In practice, in case of bidirectional link, two terminals (transceivers), placed at a certain distance and each directly visible from the other, are used. However, this transmission technique has a series of drawbacks, among which are the problems connected with scintillation, difficult mechanical alignment
As it is known, scintillation originates from the fact that the laser beam through cells of turbulence breaks up into several beamlets which, when received, can be summed with different phases so as to produce constructive or destructive interference effect. The final effect results in amplitude variations on the reconstructed signal. The higher frequency components of these variations are usually below one kHz and hence they do not directly affect the demodulation of the carried information, but they cause the received signal strength to periodically fall below the sensitivity threshold of the detector.
The second problem arises from the possible mechanical misalignment between the transmitter of one terminal and the receiver of the further terminal, caused by the unavoidable pointing imprecision and possible vibration of supports. In order to overcome this problem, the beam should have a certain divergence so that the beam angle is always wider than the amplitude of the expected vibrations. On the contrary, for optical power budget reasons, the divergence should be as small as possible.
A further problem arises from the radiated power level that, for the link stability, should be as high as possible, whereas for safety reasons it must be kept within certain limits.
The scintillation problem due to the atmosphere has been faced for the first time during the celestial observations and proper countermeasures, for instance based on the adaptive optics, have been already studied for this field. However, as far as light transmission for telecommunication purposes is concerned, the best known solution to solve the scintillation problem is described in U.S. Pat. No. 5,777,768 that also considers the problem of keeping transmission power low. The basic idea of the referenced patent, which is deemed to be the closest prior art, consists in transmitting through a plurality of apertures and/or using a plurality of laser transmitters. The plurality of apertures (and/or the plurality of transmitters) are distributed around the receiving telescope thus creating, from the scintillation view point, a plurality of space-diversity paths.
Since the transmission beams are slightly diverging, they can be made overlapping at the receiver. Under this condition, the receiver sums up all the beams received from the different apertures and, in principle, a destructive interference occuring on one beam may occur on another beam with very low probability. As a consequence, the higher the number of transmitting apertures, the smaller the amplitude variations on the received signal will statistically be. The fact of utilizing several apertures and/or several transmitters also reduces the transmission power for each aperture and therefore it goes in the direction of more easily meeting the safety requirements.
Among the drawbacks of the solution described and claimed in U.S. Pat. No. 5,777,768 is the fact that all the transmitting telescopes must be separately aligned and pointed at the receiver. The alignment accuracy, in turn, mostly depends on the mechanical stiffness of the support between the small transmitters and the big receiver. Since, as said above, the greater the number of apertures (and/or transmitters), the greater the reaction to scintillation will be, the greater the number of apertures, the bigger the alignment problems. The situation may lead to a long, inconvenient and costly waste of time for the alignment during installation and to the need to utilize, for the construction of the support, materials which are particularly strong, insensitive to temperature changes and hence expensive materials, able to ensure an alignment for a long time.
Lastly, as far as the divergence control is concerned, the prior art provides for the use of a different diffuser comprised of a glass casing containing a very large number of microsphere immersed in a fluid. This technique proves to be efficient enough but rather complicated.